1. Field of the Invention
The present invention relates to a supergain array antenna system and a method for controlling the supergain array antenna. More particularly, it relates to a supergain array antenna system that is compact and can provide a high directional gain and a method for controlling the supergain array antenna.
2. Description of the Related Art
In general, if an array antenna is downsized, the gain thereof will be reduced because the aperture area (aperture length) thereof is also reduced. However, such a gain reduction can be suppressed if antenna elements are packed in the reduced area (length) at narrow intervals and particular phase relation and amplitude relation are given to the elements. Antennas having the gain reduction thus suppressed are known as supergain antennas. A supergain antenna has a directional gain much higher than normal, and the principle thereof has been known since along time ago. For example, such a supergain antenna is described in “A new approach to the design of Super directive aerial arrays” by Bloch A, Medhurst A and Pool S (proc., Inst., Electr., Eng., 100, Part III, 67, p. 303 (September 1953)) and “Antenna Engineering Handbook” edited by the Institute of Electronics, Information and Communication Engineers, p. 211 (1980). However, it has not been put into practical use because of its severe physical constraints or the like as described below.
FIG. 9(a) shows a configuration of an array antenna. The array antenna shown in FIG. 9(a) comprises four antenna elements A-1–A-4. Signals received by the four antenna elements A-1–A-4 are output after RF (radio frequency) synthesis.
FIG. 9(b) shows a directional gain versus direction (referred to as a directivity pattern) of the array antenna thus arranged.
If a normal in-phase synthesis is applied to the array antenna having a narrow element interval (for example, about a quarter of a wavelength λ, which is abbreviated as λ/4, hereinafter) as shown in FIG. 9(a), the directional gain is reduced as the element interval decreases. That is, if a normal in-phase synthesis is applied to the array antenna having a narrow element interval, the directional gain is reduced as the element interval decreases as shown by broken lines in FIG. 10. The directivity pattern and a return loss (S11) in this case are shown in FIGS. 9b and 9c, respectively.
On the other hand, as shown in FIG. 11(a), a supergain antenna is provided in which the antenna elements A-1–A-4 are powered with the phases thereof being inverted alternately. As is known, if such a supergain antenna includes N antenna elements (N being 2 or an integer greater than 2) and the N antenna elements are spaced at intervals close to 0, a directional gain of N2is provided. That is, as shown by solid lines in FIG. 10, two elements provide a directional gain of 22=4, three elements provide a directional gain of 32=9, and four elements provide a directional gain of 42=16. The directivity pattern and the return loss (S11) in this case are shown in FIGS. 11b and 11c, respectively. FIGS. 11b and 11c show that the supergain antenna has reduced beam width and bandwidth.
However, since the supergain antenna has an increased power radiation to an invisible region in compensation for its higher gain, it has an increased Q value. Therefore, the conductor loss in the antenna including the power supply unit is increased and the efficiency of the antenna decreases. Here, the Q value is expressed as Q=D/F, where character D indicates a directional gain and character F indicates an efficiency coefficient.
To prevent the efficiency reduction of the antenna, the antenna and the power supply circuit are cooled down to reduce the conductor loss. That is, in FIG. 11(a), the N antenna elements are housed in a thermostatic container and a cooling device is provided.
In addition, the supergain antenna has a reactive power in the vicinity thereof that is much higher than the radiated power. Therefore, it has an extremely narrow band.
Furthermore, phase and amplitude relations among the antenna elements required to provide a supergain is quite sensitive, and even a small phase shift could disturb the supergain condition. For example, only 1 degree of phase shift of an antenna element would result in loss of supergain. Generation of the sensitive phase and amplitude, or RF synthesis, is difficult using a power supply circuit, such as a microstrip line, because of its physical constraints (fabrication precision, stability). The difficulty becomes higher as the number of antenna elements increases.
An example of the supergain antenna using two-element helical antenna has been reported. However, it essentially requires delicate adjustment of a matching circuit required for RF synthesis, and therefore, it is difficult to use a large number of elements in the supergain antenna. This is described in “High-Tc Superconducting Small Antennas” by K. Itoh, O. Ishi, Y. Nagai, N. Suzuki, Y. Kimachi and O. Michikami (IEEE Trans. Applied Superconductivity, Vol. 3, No. 1, March 1993). Thus, no example of a multi-element array that provides a supergain has been reported.
Beside, if fixed phase and amplitude are given by the power supply circuit (RF synthesis), the whole antenna system would have a narrow band, and the system including a receiver would also have a narrow band. As a result, a problem arises in that the antenna cannot be applied to a wide band communication system.
Furthermore, there is a significant problem concerning directivity synthesis. Since the supergain array antenna has the antenna elements spaced at quite narrow intervals, the elements are electromagnetically strongly coupled to each other and therefore have non-uniform directivities. To the contrary, in an array antenna having an element interval of about λ/2 or more, elements other than those at both ends have a substantially uniform directivity, and directivity synthesis can be implemented without hindrance. Since the supergain synthesis requires such a phase relation that adjacent elements have inverted phases, the directivity of each of the elements is an important design factor. That is, to provide phase and amplitude that realize a supergain, the directivity of each element in operation is needed.
Mathematically, by assuming a nondirectional antenna, phase and amplitude that realize a supergain can be found. However, the elements are electromagnetically coupled to each other in actual, and therefore, the supergain cannot be realized if the values found are applied to a directional antenna.
According to the conventional synthesis method (RF synthesis) using a power supply circuit, directivities of mounted elements connected to the power supply circuit that gives operation conditions, that is, phases and amplitudes to the elements cannot be measured, and therefore, supergain synthesis taking the element directivities into account is difficult.
As described above, it has been technically difficult to design multi-element, high-precision and wide-band supergain antenna system hardware taking into account all of a plurality of design factors.